Use of electro osmosis plus freezing in construction of underground storage tanks



Jan. 28. 1969 v c. M. Huoems, JR.. ETAL 3,

USE ELECTROOSMOSIS PLUS FREEZING IN CONSTRUCTION OF UNDERGROUND STORAGE TANKS Filed Oct. 15, 1965 Sheet FIGURE I FIGURE 2 S R m j. IDN E G E V DMD m HCH MML C.B.E Y B ATTORNEY Filed Oct. 15, 1965 Sheet 2 Jan. 28. 1969 c. M. HUDGINS, JR. ETAL 3,424,662

USE OF ELECTED-OSMOSIS PLUS FREEZING IN CONSTRUCTION OF UNDERGROUND STORAGE TANKS FIGURE 3 POWER SUPPLY 'L LL.

FIGURE 4 1 REFRIG- F ERANT SUPPLY I f m 9 VVATER I SCHARGE ELL INVENTOR c. M. HUDGINS, JR. s. M. CASAD BY E. L. HADEN v I 4 FIGURE5 FIGURE 6 ATTORNEY United States Patent 3,424,662 USE OF ELECTRO-OSMOSIS PLUS FREEZENG IN CONSTRUCTION OF UNDERGROUND STORAGE TANKS Charles M. Hudgins, Jr., Burton M. Casad, and Elard L. Haden, Ponca City, Okla., assignors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware Filed Oct. 15, 1965, Ser. No. 496,644 U.S. Cl. 204-180 12 Claims Int. Cl. Btilk /00 This invention relates to a method of stabilizing soil prior to excavating for underground storage of liquefied normally gaseous materials which have been liquefied by cooling to a temperature below about 20 F.

In recent years, underground storage of liquefied normally gaseous materials has become of increasing importance. Naturally occurring porous formations such as exhausted gas reservoirs, or gas reservoirs of insufficient size to be of commercial value, located near large gas consumption centers, are frequently employed. Also, caverns in salt beds, usually prepared by solution mining methods have been used. Such wells are ideal for storage of liquefied gases during low consumption periods, and gas is used therefrom during high consumption periods. Unfortunately, there is a need for large storage capacities in areas where such ideal storage structures are not available.

Casagrande in US. Patent 2,099,328 utilizes electro osmosis to introduce aluminum ions into certain clayey soils to stabilize such soils prior to excavation. That is, aluminum ions from the aluminum electrode are carried by the electric current through the soil and replace the sodium ions contained in the colloidal fraction of the clay. This method has three serious limitations. First, again a particular soil structure is required, and, secondly, since substantially all the water is removed from around the anode, additional water must be added. Lastly, if used exclusively for said stabilization while excavating for liquefied gas storage, the initial liquefied gas placed in the chamber would be lost until the Water in the walls of the excavated well is frozen. Freezing after completing the evacuation would be time consuming and would probably cause the excavation wall to spall. Except for the aluminum anode, the stabilization is reversible, and the soil becomes unstable when the current is removed.

Underground storage for LNG (liquefied natural gas), LPC (liquefied petroleum gas), usually propane or butane, and ammonia (liquefied) has a number of advantages over surface storage including lower cost, and, primarily, greater safety. The choice of location for these storage facilities is usually dictated primarily by proximity to major distribution centers. In ammonia storage, however, the choice is generally near the source of supply, i.e., the ammonia synthesis plant. In a large number of instances, it appears that optimum locations, i.e., low cost land in good location, will be in marshy lands (shorelines or riverbanks) adjacent cities or snythesis plants. Excavation of a large deep hole in such unconsolidated material raises some serious operational problems in stabilizing the sides of the hole. Due to the large amount of water present, electro-osmosis is not completely satisfactory, and even with such systems, the process is reversible and problems are encountered upon the reduction of current. One proposed technique has been to drill or drive pipes into the ground to the desired depth and in the desired geometry and to circulate refrigerant to freeze the earth, thereby building strength in the walls. However, the earth nearest the freeze tube freezes first and reduces the thermal conductivity, thus making this 3,424,662 Patented Jan. 28, 1969 approach time consuming. Delays of several months can be anticipated for the freezing to occur.

It is an object of this invention to provide an underground storage for liquefied normally gaseous materials. It is a particular object of this invention to provide an underground storage for such gases in marshy lands.

These and other objects of this invention are accomplished by utilization of electro-osmosis with freezing. In a particular aspect, the soil is first stabilized by electroosmosis followed by simultaneous freezing and electroosmosis.

To further illustrate the invention, reference will be made to the drawings in which:

FIGURE 1 is a schematic top view pipe arrangement for stabilizing a vertical wall cavern;

FIGURE 2 is a side elevation taken along the section 22 of FIGURE 1 showing two of the freeze pipes locations in the ground; 7

FIGURE 3 shows an alternative arrangement of the freeze pipes in the ground;

FIGURE 4 is a schematic diagram of the electrical control system;

FIGURE 5 is a side view of a typical freeze pipe; and

FIGURE 6 is a side view of a typical perforated pipe as will be later described.

Referring now to the figures, alternately freeze pipes 3 and perforated pipes 1 and 2 are placed in the ground around that portion to be excavated. Generally, these tubes or pipes are of iron or steel, but it will be obvious that any material capable of conducting current will be suitable. Casagrande, supra, has described some advantages of aluminum anodes. In FIGURE 1, a circular arrangement is shown; however, it is obvious that other arrangements can be used. Between each freeze pipe 3 is a perforated pipe arranged so that pipes 1 and pipes 2 alternate between pipes 3, that is on either side of pipe 3 is a pipe 2 and on the other side a pipe 1. The length of these pipes will, of course, depend upon the depth of the desired excavation. The spacing of the pipes will depend upon the nature of the soil and amount of moisture presentthe important factor being spaced close enough that current will freely flow at economical potentials. See the paper of Dr. Leo Casagrande, titled, Electro- Osmotic Stabilization of Soils, presented at a meeting of the Structural Section of the Boston Society of Civil Engineers, held on Jan. 10, 1951, and the Casagrande patent supra. In general, freeze pipe spacing of 10 to 40 feet apart, and preferably 10 to 20 feet, should give satisfactory results. However, lesser or greater distances can frequently be employed. These pipes can be placed in the soil by any convenient means, however, in the marshy lands, driving the pipes into the ground is a satisfactory method.

The freeze pipes 3 can be of any convenient construction that would permit circulation of a refrigerant through same. FIGURE 5 illustrates one convenient means. In this case, the pipe 3 comprises an outer shell 4 and an inner tube 5. A refrigerant supply 6 is connected so that refrigerant can be circulated through the inner tube and withdrawn through the annular space between tube 5 and shell 4. It would be obvious that the flow direction could be in the opposite direction, and, in some circumstances, this might be desired. It is also within the skill of the art to provide other freeze means such as, for example, a cooling coil within the pipe 3.

The pipes 1 and 2 (see FIGURE 6) comprise a perforated outer shell 7 and an inner tube 8. The inner tube 8 is operably connected to pump 9 arranged to remove water from the pipes 1 and 2 and discharge the water via conduit 10. The pump 9 can be of any convenient type which will lift water the desired distance. For example,

in a shallow excavation a vacuum pump would suffice. If the excavation is too deep for a vacuum pump, a piston lift pump or deep well bottom hole pump can be utilized. In the latter case, the pump would be pulled prior to final freezing. In some cases, a gas such as air can be forced down through conduit 8 and the gas utilized to flow the water out. Again, the particular pump arrangement will be selected to fit the individual situation as will be known tothose skilled in the art.

Direct current power supply 11 having positive terminal 12 and negative terminal 13 is operably connected to conduits 14, 15 and 16 via switches 17, 18, 19 and 20. The switches are arranged so that freeze pipes 3 are connected in parallel to anode 13 via switches 18 and 19. Perforated pipes 1 are connected in parallel to negative source 13 via switch 17 or to the positive terminal 12 via switches 18 and and perforated pipes 2 are similarly connected to the negative terminal via switches 17 and 20 or the positive terminal via switches 18 and 20.

In operation, ordinarily no refrigerant is initially circulated through pipes 3, but rather initial stabilization is obtained by electro-osmosis utilizing pipes 3 as the anode and pipes 1 and 2 as cathodes. Thus, switch 18 will be connected to the positive 12 of the power supply, whereas 17 and 20 will be connected to the negative 13 of the power supply. Thus current will flow from pipes 3 to pipes 1 and 2 carrying water to pipes 1 and 2. This water will flow through the perforations and be removed via pump 9. This will continue until initial soil stabilization is established and excavation can begin. While in the most usual situation, water migrates from the anode to the cathode and the above-described arrangement is usual. However, it has been reported in the literature that with certain soils, the migration is from cathode to anode. This can readily be compensated for, by merely reversing the direction of current flow.

After initial stabilization has been obtained, switch 19 is put in the open position, and switches 18 and 20 are closed so as to make pipes 2 the anode. At this time, refrigerant is passed from supply 6 to pipes 3. The current flowing from 2 to 1 will then carry moisture past pipes 3 where it loses heat and begins to freeze. The freeze wall will extend toward pipes 1. However, before the ice wall reaches pipes 1, the current is reversed by connecting conduit 14 via switch 17 to the anode 12 and connecting conduit 15 to the cathode via moving switch 18 to the cathode 13. The movement of water then reverses and the freeze moves toward pipes 1. This reversing of flow direction is continued until the freeze wall is completed. Such reversing of flow direction has the additional advantage of providing uniform corroding of the pipes since, it is well known that the anode will lose metal toward the cathode. By this time the excavation should be completed and the cavern covered and equipped for storing and removing liquefied normally gaseous materials. Since thesematerials remain liquid at very low temperatures, the need for refrigerant circulation in pipe 3 is no longer needed, and the circulation is stopped. Prior to complete Ifreeze of the soil around pipes 1 and 2, the pumps are removed.

Where the liquefied gas is immiscible with Water, as suming it is physically strong enough to stay in place during excavation, no prefreezing of the bottom of the cavern is required. However, in the case of miscible fluids such as liquefied ammonia, the bottom will also have to be prefrozen to prevent contamination of the fluid to be stored. FIGURE 3 illustrates one method of accomplishing this. The pipes 1, 2 and 3 are driven into the ground to circumscribe an inverted cone. The bottom of the pipes will be close enough together that the bottom as Well as the walls will be frozen. The bottom can also be frozen, for example, in a vertical wall excavation, by placing freeze coils in the bottom of the excavation either with or without electro-osmosis. In case of extremely marshy or wet locations, then the combination electroosmosis plus freeze tubes can advantageously be employed.

From the foregoing description, it can be seen that the method of this invention can be adopted to many applications and the particular time cycles will depend upon the nature of the soil surrounding the portion to be excavated. For example in extremely dry soils, it would be advantageous to pass water into the perforated pipe in order to provide suflicient moisture to form the ice wall. In some cases, refrigerant may be started immediately and pipes 3 used as the anode only until initial freezing around said pipes raises the resistance to current flow to uneconomical voltage levels or even the initial flow can be from pipes 1 to 2 or 2 to 1.

To further illustrate the invention the following examplc is given.

SPECIFIC EXAMPLE A cavern is to be prepared for storage of cryogenic material, such as liquid natural gas, by excavation. The soil is in a marshy area; however, there is no problem of bottom water entering the excavated portion. The cavern is approximately 10 0 feet in diameter and 20 feet deep or a circumference of about 300 feet. Ten freeze pipes, each 20 feet long are driven into the marshy earth at 30 feet apart on the circumference of the area to be excavated. Intermediate each freeze pipe is driven a 20 foot long perforated pipe. A direct current power supply is connected to these pipe-s as shown in FIGURE 4. A refrigerant supply is connected to the freeze pipes so that refrigerant can be circulated through same as in FIGURE 5. A means for removing water from the perforated pipes is connected to each such pipe as shown in FIGURE 6.

Initially sufficient power from the power supply is applied to the freeze pipes as the anode and the perforated pipes as the cathode to overcome the soil resistance. It will be understood by those skilled in the art, that this power requirement will be determined by the moisture content, dissolved salts and nature of the soil. This current will continue until initial stabilization is obtained as determined by periodic soil checks. After initial stabilizaton, refrigerant is pumped through the freeze pipes at a temperature below the freeze point of the surrounding moisture, preferably at least 10 degrees below said freeze point. Again each soil will have a freeze point dependent upon the concentration of dissolved salts, etc. and can readily be determined by those skilled in the art by conventional means. The current is then switched soas to make every other perforated pipe a cathode and the remaining perforated pipes the anode. It will be noted that when the freeze pipes were the anodes, the cathodes were approximately 15 feet from said anodes. However, when the switch is made, the anodes and cathodes are spaced approximately 30 feet apart. As a consequence the power requirements will be increased. As previously explained, as the water migrates past the freeze pipes a wall of ice begins to form with said wall progressing in the direction of water flow at a faster rate than in the opposite direction. Therefore after the wall progresses about 5 feet, the direction of current flow is reversed. This alternate reversing the current flow is continued until the entire cylinder around the proposed cavern is frozen.

Immediately after the initial stabilization, excavation can begin. If this initial excavation is started near the proposed outer limits of the excavation, that is nearest the cylinder wall, the water of migration will be forced to pass the freeze pipes outside the proposed cavern, thus forming the ice walls outside the proposed excavation. By the time the excavation is completed, the ice cylinder will generally be completed. The excavation is then covered with steel, concrete or other desired construction material and can be anchored to the ice wall by tieing to the pipes which are frozen in place. The liquefied gas, which will be at a temperature below the freeze point of the moisture is introduced into the storage cavern thus formed and the refrigerant shut off.

In the above example, it is assumed the Water will migrate from the anode to the cathode. In those soils where water will migrate in the opposite direction, the initial fiow of current is reversed so that the perforated pipes are the anode and the freeze pipes are the cathode since it is desirable in most cases to remove excess water as it accumulates in the perforated pipes. Where the soil is relatively dry, the removal of water can be stopped prior to complete freezing and in some cases, the perforated pipes can be used as a means of introducing water into the soil to provide the desired ice wall. Those skilled in the art will see other modifications which can be made in utilizing this invention dependent upon the particular said conditions at the desired location of the excavation.

Having thus described the invention, we claim:

1. A method of stabilizing soil for excavation which comprises introducing into said soil a plurality of freeze pipes circumscribing the area to be excavated, introducing intermediate said freeze pipes perforated pipes, passing current from a portion of said pipes to at least part of said perforated pipes to establish electro-osmotic stabilization and passing refrigerant through said freeze pipes until the area to be excavated is circumscribed by a frozen cylinder.

2. The method of claim 1 wherein the soil is initially at least partially stabilized by passing current from said freeze pipes to said perforated pipes, thereafter circulating refrigerant through said freeze pipes and then passing current from alternate perforated pipes to the remaining perforated pipes so that current carries moisture past said freeze pipes.

3. The method of claim 2 wherein current passing between perforated pipes is reversed periodically.

4. The method of claim 1 wherein current passing between perforated pipes is reversed periodically.

5. The method of claim 1 wherein excess water is removed from said perforated pipes.

6. The method of claim 2 wherein excess water is removed from said perforated pipes.

7. The method of claim 3 wherein excess water is removed from said perforated pipes.

8. A method of excavating unstable soil for storage of liquefied natural gases which comprises initial stabilization of the earth circumscribing the earth to be excavated by electro-osmosis, and thereafter maintaining electro osmosis and freezing the earth circumscribing the earth to be excavated during said excavation.

9. An apparatus for preparing an excavation in nonstable soil suitable for storage of liquefied normal gaseous materials, said apparatus comprising in combination a plurality of freeze pipes circumscribing the earth to be excavated, a refrigerant supply, means for passing refrigerant from said supply to said freeze pipe, a plurality of perforated pipes intermediate said freeze pipes, means for withdrawing water from said perforated pipes, a direct current supply, means for electrically connecting and disconnecting said freeze pipes to the anode of said current supply, means for electrically connecting and disconnecting said perforated pipes to the cathode of said current supply, and means for alternately connecting every other perforated pipe to the anode and cathode of said power supply and means for alternately connecting the remaining perforated pipes to the cathode and anode of said power supply, said means for alternately connecting said perforated pipes being reverse to each set of said pipes.

10. The apparatus of claim 9 wherein all of said pipes are vertical.

11. The apparatus of claim 10 wherein said freeze pipes are spaced 10 to 30 feet apart.

12. The apparatus of claim 9 wherein all of said pipes are at an angle such that the lower ends of said pipes are closer together than are said pipes at their upper ends.

References Cited UNITED STATES PATENTS 2,099,328 11/1937 Casagrande 204- 2,625,374 1/1953 Neuman 204 2,806,818 9/1957 Howard 204-430 2,932,170 4/1960 Patterson et al. 61.5

FOREIGN PATENTS 432,253 7/ 1935 Great Britain.

495,518 11/ 1938 Great Britain.

512,762 11/ 1939 Great Britain.

OTHER REFERENCES Casagrande: Electro-Osmotic Stabilization of Soils," Structural Section of the Boston Society of Civil Engineers, Jan. 10, 1951.

JOHN H. MACK, Primary Examiner. A. C. PRESCOTT, Assistant Examiner.

US. Cl. X.R. 61-36 

1. A METHOD OF STABILIZING SOIL FOR EXCAVATION WHICH COMPRISES INTRODUCING INTO SAID SOIL A PLURALITY OF FREEZE PIPES CIRCUMSCRIBING THE AREA TO BE EXCAVATED, INTRODUCING INTERMEDIATE SAID FREEZE PIPES PERFORATED PIPES, PASSING CURRENT FROM A PORTION OF SAID PIPES TO AT LEAST PART OF SAID PERFORATED PIPES TO ESTABLISH ELECTRO-OSMOTIC STABILIZATION AND PASSING REFRIGERANT THROUGH SAID FREEZE PIPES UNTIL THE AREA TO BE EXCAVATED IS CIRCUMSCRIBED BY A FROZEN CYLINDER. 